Fully automated robotic control has been utilised in many industrial applications, but there are still many applications where such a technique is not sufficiently mature to be applied. The main reason is the lack of an intelligent and stable control system that can operate effectively when unscheduled tasks and unpredictable disturbances appear. Another reason is that the software modelling of a task and a workspace is usually very complex so that in many circumstances such modelling is not practical. Where automated robotic control is not practical, the conventional human-in-the-loop control method is the alternative because the human operator in the control loop is capable of generating control signals adaptive to task and workspace variations. A human operator creates a control loop even where the only feedback to him is visual--he can adapt his control signals according to what he sees the robot doing.
The control input device for a human-in-the-loop system varies according to the human engineering factors and the nature of the applications. For resolved rate control and resolved position control, a commonly used input device is a passive hand controller comprising a pair of three degree-of-freedom joysticks, one for translational control and one for rotational control.
Where the gripper of the manipulator experiences constrained forces, it is essential that the constrained forces be controlled effectively. One known method to control forces in the human-in-the-loop system is to measure the forces and display them graphically or numerically. The operator watches the displayed forces as he/she regulates the manipulator through the passive hand controller. This is called force feedback via vision. This method can only be used in a very slow control system because of the time delay in displaying the force and the time lag in the vision feedback and interpretation process. Other disadvantages of this method are that it requires the operator to watch the manipulator operation as well as the displayed forces, hence it may divert the operator's attention and cause excess operator strain.
Another method is to reflect the forces encountered by the manipulator through a hand controller to provide force feel for the operator. The operator can then control the encountered forces by adjusting the hand controller position, which is interpreted as the manipulator command signal. This method does not require a visual display of the forces, hence the operator can concentrate on the motion of the manipulator. Master-slave control has been the conventional method to reflect the forces encountered by the manipulator in the last thirty years. The method requires two identical or scaled manipulators, one (the master) being moved by an operator as a hand controller for generating command signals and reflecting forces, and the other for performing the task. Master-slave control uses the discrepancies between joint positions of the master manipulator and the corresponding joint position of the slave arm to generate force signals. This system is expensive and requires a large workspace. The master arm (control input device) kinematics are unique to a particular slave arm, hence the master arm cannot be used with other types of manipulators. Furthermore, the mechanical system is very complicated and can only handle small payloads (up to about 50 pounds).
The characteristics of the master-slave system do not meet present advanced manipulator control needs, for example the control of large robots in space.
Recently, two new force reflecting hand controllers have been developed. These are the six Degree Of Freedom (DOF) Universal Force Reflecting Hand Controller of JPL (Jet Propulsion Laboratory), and the 9-String 6-DOF Force Reflecting Hand Controller of the University of Texas at Austin. The former hand controller consists of a set of pulleys and cables to transmit joint motor torques to the handle. The latter hand controller uses cables and cylinders to transmit joint motor torques to the handle. Both hand controllers suffer the drawback that the dynamic characteristics of the cables, pulleys and cylinders disturb the force feedback loops. Other drawbacks of these hand controllers are that the torque transmission mechanisms have significant friction, the mechanical system and the software are complex, and the physical sizes, though small as compared with the master arm of the master-slave system, are still relatively large.
Further, in no known active (i.e., force reflecting) human-in-the-loop control loop is it possible to emulate a smooth passive (i.e., non-force reflecting) human-in-the-loop control loop. For some applications, for example in manual augmented (resolved rate) control of the remote manipulator of NASA's space shuttle, this feature is essential.
Consequently, there remains a need for a machine control loop capable of reflecting forces encountered at the operative part of the machine in a hand controller that avoids the problems and the drawbacks of known systems.